1. Field of the Invention
The present invention relates to antennas, in particular, to patch antennas used in global navigation satellite systems (GNSS).
2. Description of the Related Art
Patch antenna systems are used in different radio electronic devices. They are widely applicable in ground satellite navigation systems (GPS, GLONASS, Galileo etc.), with the help of which a position of an object can be quickly and accurately determined at any point of the world. One of the main reasons for reduced GNSS positioning accuracy of land objects is related to receiving not only the line-of-sight satellite signal but also signals reflected from surrounding objects, and especially from the Earth's surface. The strength of such signals depends directly on the antenna's directional diagram (DD) in the rear hemisphere.
A right-hand circularly polarized signal (RHCP) is used as a working signal in navigation systems. Signals reflected from the Earth's surface, when there are no major surface features, are mostly left-hand circularly polarized signals (LHCP). This also holds true for signals of satellites that are at an angle over the horizon that is higher than Brewster's angle, that is, for typical soils, about 10-15 degrees over the horizon plane. Considering this, a GNSS antenna systems need to have a lower DD level in the rear hemisphere, and primarily, a lower component of the LHCP (cross-polarized) signal. A reduction in antenna weight and dimensional characteristics is also required.
The simplest method of reducing DD level in the rear hemisphere is mounting the antenna directly on a metal or impedance ground plane. However, this results in increasing antenna dimensions. Another method is the use of an additional antenna, the field of which is anti-phase-added to the main antenna field. This provides a reduction in the radiation level of the rear hemisphere. U.S. Pat. No. 6,836,247 B2 shows a design of a circularly-polarized antenna in the form of two patch (MP) radiators axial-symmetrically disposed one under another (see FIG. 1a). A ground plane of the top radiator is under a radiating patch, and a ground plane of the bottom radiator is over the radiating patch. In an isolated cavity of the ground planes, there is a low-noise amplifier (LNA). The top radiator is actively excited by pins; the bottom radiator is passively excited. Such a design provides a noticeable reduction in LHCP field only in the vicinity of anti-normal direction, while the antenna's vertical dimension still remains very large.
Modern high-precision positioning receivers employ signals of different frequencies. Operating GPS frequencies are 1575 MHz (L1-band), 1227 MHz (L2-band) and a frequency of 1175 MHz (L5-band) was recently added. GLONASS and GALILEO satellite systems also broadcast some operating frequencies. In total, the operating frequencies of GNSS systems lie in two frequency ranges: low-frequency (LF 1165-1300 MHz) and high-frequency (HF 1525-1605 MHz). Antennas of high-precision navigation devices need to operate in the both frequency bands. In most cases, antenna designs include two radiators operating at their own frequencies. U.S. Pat. No. 6,836,247 B2 describes a dual-band stacked antenna (FIG. 1b). Such a combined antenna includes two active MP radiators disposed one over the other, and two passive ones. The radiating patch of the low-frequency radiator serves as a ground plane of the high-frequency radiator. Bandwidth expansion of each radiator is normally attained by increasing the distance between the radiating patch and ground plane, i.e., increasing the thickness of MP radiator. Note that an increase in LF radiator thickness results in increasing the distance between active and passive HF radiators. This, in turn, causes reduction in their coupling and excitation level of the passive radiator, and, hence in the antenna's less efficient operation.
The proposed technical solution is intended at solving cross-polarized (LHCP) field suppression problems in a wide angle sector of the rear hemisphere, enhancing the operation of the passive HF radiator in the dual-band antenna, and reducing antenna dimensions.